A liquid crystal display device has been used more and more in watch and electronic calculator as well as various measuring instrument, panel for automobile, word processor, electronic note, printer, computer, television, etc. Representative examples of liquid crystal display system include TN (twisted nematic) system, STN (super-twisted nematic) system, DS (dynamic scattering) system, GH (guest-host) system, and FLC (ferroelectric liquid crystal) system, which allows high speed response. Referring to driving system, multiplex driving has been more common than the conventional static driving. Further, simple matrix system has been recently put into practical use. Moreover, active matrix system has been put into practical use.
As the liquid crystal materials to be incorporated in these liquid crystal display devices there have been synthesized a very large number of kinds of liquid-crystalline compounds to date. These liquid-crystalline compounds are used depending on their display systems driving systems or purposes. However, the requirement for improvement of the properties of liquid crystal display device (enhancement of display quality or increase of the size of display) has been growing more and more. In order to satisfy these requirements, the development of now liquid crystal compounds are under way.
A liquid crystal compound is composed of a central skeleton moiety generally called core and a terminal moiety at both ends thereof. In general, the majority of the ring structure constituting the core moiety of a liquid crystal compound is occupied by 1,4-phenylene group (which may be substituted by one or two halogen atoms, cyano groups, methyl groups, etc.) and trans-1,4-cyclohexylene group. However, a liquid-crystalline compound merely made of 1,4-phenylene group and trans-1,4-cyclohexylene group is limited in its kind or properties. As a matter of fact these components cannot sufficiently meet the foregoing demands.
Besides the ring structures such as 1,4-phenylene group and trans-1,4-cyclohexylene group, heterocyclic groups such as pyridine-2,5-diyl group, pyrimidine-2,5-diyl group and 1,3-dioxane-trans-2,5-diyl group, condensed rings such as trans-decahydronaphthalene-2,6-diyl group, naphthalene-2,6-diyl group, tetrahydronaphthalene-2,6-diyl group, bicyclo[2,2,2]octane-1,4-diyl group and spiro[3,3]heptane-2,6-diyl group, etc. have been under study. However, few of these compounds have been put into practical use due to problems in production (technique, cost, etc.) and stability.
Though being a ring structure which has long bean reported, naphthalene-2,6-diyl group among these condensed rings is little known for properties other than liquid crystallinity (phase transition temperature), particularly nematic liquid crystallinity. (For smectic liquid crystal, it has recently been reported that an optically active alcohol ester of naphthalenecarboxylic acid exhibits interesting properties as a ferroelectric liquid crystal.)
An ordinary liquid crystal compound is terminated by a chain (side chain) group at one and thereof at least. A so-called p-type liquid crystal the dielectric anisotropy of which is positive is mostly terminated by a polar group at the other end thereof.
In order to lower the driving voltage in TN or STY display system a so-called strong P-type compound (the dielectric anisotropy of which is positive and great) is required. For this purpose, a compound terminated by cyano group at the molecular end thereof and having one or more fluorine atoms in the same direction per molecule is normally used. As a naphthalene derivative there has been reported only a compound having phenylnaphthalene skeleton (GB2271771A: Citation (a)). There is no reference to physical properties and application.
As the foregoing P-type compound to be used in active matrix driving there is used a compound containing as a polar group fluorine atom, fluoroalkoxyl group or fluoroalkyl group alone. As a naphthalene derivative there is described only a compound having phenylnaphthalene skeleton in GB2227019B (Citation (b)) and the above Citation (a). However, there is little reference to specific physical properties. Further, there is no reference to application to active matrix system.
In general, a liquid-crystalline compound which is a naphthalene derivative mostly exhibits a poor miscibility with other liquid crystal compounds. It is thought effective to introduce side substituents (preferably fluorine atom in particular) into the naphthalene skeleton for the purpose of improving the miscibility of the naphthalene derivative. The substitution by fluorine atom is thought effective also if an end polar group is directly introduced into the naphthalene ring in the application to the foregoing active matrix system. Some examples of such a fluoronaphthalene derivative are shown in the foregoing Citation (a). However, there is little reference to production process, not to mention physical properties. Thus, it is by no means thought that these exemplified compounds are actually produced compounds. Further, it cannot be presumed what properties these compounds have. No compounds having a structure having a fluoroalkoxyl group or fluoroalkyl group directly connected to naphthalene ring as a polar group have been known.
It has been known that a liquid crystal compound having an alkenyl group instead of an alkyl group, which is normally used as a side chain moiety, exhibits excellent improvements such as improved liquid crystallinity, reduced viscosity and improved sharpness in display. However, these alkenyl groups are mostly introduced directly connected to cyclohexane ring. Thus, no compounds having alkenyl groups introduced in naphthalene ring have been reported.
Similarly, no naphthalene derivatives having as side chain alkoxylalkyl group, fluoroalkyl group, fluoroalkenyl group, fluoroalkenyloxy group, etc. have been reported.
As groups connecting the ring structures in the core moiety of a liquid crystal compound, there have been known many divalent organic groups besides single bond and 1,2-ethylene group (—CH2CH2—).
Liquid crystal compounds having 1,4-butylene group or 1,2-propylene group are known to have a low melting point and an excellent miscibility with other liquid crystal compounds as compared with the corresponding liquid crystal compounds having single bond or ethylene group. However, no liquid-crystalline compounds having 1,4-butylene group or 1,2-propylene group have been known among naphthalene derivatives.
It has been reported that a liquid crystal compound having difluoroxymethylene group (—CF2O—, —OCF2—) or difluoroethenyl group (—CF═CF—) exhibits a low viscosity and thus is effective for the enhancement of response. However, no such naphthalene derivatives have been known.